It is common to mount one or more fluid level sensors on rigid blood reservoirs to trigger warnings (alerts or alarms) when the fluid level in the reservoir exceeds or drops below various levels. An "alert" is an audible and/or visible warning to the perfusionist or other medical practitioner that, for example, the liquid level in the reservoir has dropped below a specified level, with the perfusionist or other medical practitioner manually taking any desired or necessary steps in response. An "alarm" is an audible and/or visible warning plus an automatic response by the equipment, for example, stopping a perfusion pump. One preferred technique is to provide an "alert" for a first low level of liquid, and an "alarm" when the liquid drops below a second, even lower level.
Examples of ultrasonic blood level sensor have been available under the trade designations "SARNS.TM. 8K.TM." and "SARNS.TM. 9K.TM." level sensors from Minnesota Mining and Manufacturing Company, St. Paul, Minn.; "JOSTRA HL20" level sensor from Jostra AB, Lund, Sweden; "COBE/CPC.TM. SYSTEM" level sensor from Cobe Laboratories, Inc., Lakewood, Colo.; and "STOCKERT-SHILEY CAPS.TM." and "STOCKERT LEVEL DETECTOR II" from Sorin Biomedica SpA, Saluggia, Italy, or Sorin Biomedical Inc., Irvine, Calif. Other sensors may also be mounted on the walls of such reservoirs, for example, blood gas sensors.
Typically, rigid blood reservoirs have housing walls formed of transparent and ultrasonic-compatible material, such as polycarbonate. Frequently, the reservoirs have generally round cross sections or other non-flat wall sections. Examples of such rigid blood reservoirs have been available from Minnesota Mining and Manufacturing Company, St. Paul, Minn., under the trade designations "SARNS.TM." venous reservoir and "SARNS.TM." integral venous/cardiotomy reservoir. Rigid blood reservoirs are disclosed, for example, in co-assigned U.S. Pat. Nos. 5,149,318; 5,152,964; 5,254,080; 5,282,783; 5,304,164; and 5,403,277, and co-assigned U.S. patent application Ser. No. 08/142,809, filed Oct. 25, 1993.
One prior level sensor mounting pad has been available under the trade designation "SARNS.TM. Level Sensor Pads (Part No. 16432901)" from Minnesota Mining and Manufacturing Company, St. Paul, Minn. Those pads were packaged in pairs, to facilitate use with "alert" and "alarm" sensors, along with a conventional gel which is placed between the sensor and the housing wall of the reservoir. That pad was relatively rigid and included a bayonet-type mounting ledge for receiving bayonet-type locking tabs along the outer side of the pad, and an adhesive-coated foam tape along the inner side of the pad. As used herein, "outer side" refers to the side of the pad that faces away from the reservoir in use of the pad, and "inner side" refers to the side of the pad that faces and is affixed to the reservoir. The inner side of the pad included a concavely curved inner portion extending between the upper and lower edges of the pad along an intermediate portion of the pad that was designed to conform to the round cross section of a reservoir, and flat end portions extending between the upper and lower edges of the pad along the side edges of the pad in order to conform to fiat walled reservoirs. That pad further had an opening through the pad to allow the transducer lens of an ultrasonic transducer to obtain an operational relationship with the reservoir, e.g., to contact the wall of the reservoir. Except for the foam tape, that pad was integrally molded in one continuous piece of the polycarbonate material available under the trade designation "MERLON.TM. No. 2608" from Mobay Chemical Corporation, Pittsburgh, Pa.
Another mounting pad has been available under the trade designation "Mounting Pad (Product Designation: 23-22-08)" from Shiley, Inc., Irvine, Calif., (now Sorin Biomedical Inc.). That mounting pad includes an adhesive for holding the mounting pad on the housing wall of the reservoir.
Other mounting pads have been available under the trade designation "Catalog No. 043-220-350 Level Sensor Disposable Plate" from Cobe Cardiovascular, Inc., Arvada, Colo., and the trade designation "Catalog No. 043220-300 Attachment Plate" from Cobe Laboratories, Inc., Lakewood, Colo. Those mounting pads each include an adhesive for holding the mounting pad on the housing wall of the reservoir.
One problem with many prior mounting pads is failure of the mounting pad to remain fixed to the reservoir wall during use. The result frequently is the generation of false alerts and alarms.
Typically, ultrasonic transducers are spring-loaded against the housing wall of the reservoir, with the transducer applying an equal outwardly-directed force to the mounting pad holding the transducer on the reservoir. Such outwardly-directed forces tend to lead to creep of the adhesive, leading in some cases to separation of the mounting pad from the reservoir. Over time the mounting pad "peels" from the edge of the adhesive area, which is where force tends to be concentrated in the prior designs. One response is for the perfusionist to attempt to stick the mounting pad back on the reservoir, which may work for awhile anyway. Other forces tending to separate mounting pads from reservoir walls include the weight of the transducer/sensor cable, and jostling of the transducers and cables during use.
An infra-red blood level sensor has been available under the trade designation "SARNS.TM. 7000.TM." level sensor. That level sensor was mounted with a suction-type mount.